1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to temperature control on the fixing member of the image forming apparatus.
2. Description of the Related Art
Among image forming apparatuses, each of those forming toner images on sheets and thermally fixing the toner images, such as laser printers, facsimile machines, and copying machines, includes a fixing unit. The fixing unit is a functional unit that performs a fixing process on a supplied sheet, and includes a fixing member and a heater. The fixing member includes a rotator such as a roller or a belt, and transfers heat to a sheet by bringing the rotator into contact with the surface of the sheet. The heater is designed to heat the fixing member, and normally uses a halogen lamp or induction heating.
To further reduce the power consumption in such an image forming apparatus, a reduction in the power consumption by the fixing unit is effective. In practice, the electric power required for the fixing unit to perform an operation to continuously increase the temperature of the fixing member (hereinafter referred to as the “temperature increasing operation”) and an operation to maintain the temperature at a constant level (hereinafter referred to as the “temperature adjusting operation”) accounts for a large proportion of the power consumption by such a machine.
The following technique is known as a technique for reducing the power consumption by the fixing unit: “in an operation mode that does not require the fixing process, or while a sheet that does not require the fixing process is being supplied, the fixing unit is made to stop both the temperature increasing operation and the temperature adjusting operation”. While these operations are suspended, the temperature of the fixing member drops. Therefore, in this technique, it is critical to accurately determine the time to cause the fixing unit to resume the temperature increasing operation. If the resumption of the temperature increasing operation is late, the temperature of the fixing member does not reach the value that should be maintained during the fixing process even when the time for the next fixing process has come. As a result, the thermal fixing of a toner image becomes insufficient, and the toner image is scratched, for example. Therefore, there is a possibility of a decrease in printing quality.
As techniques for accurately determining the time to resume a temperature increasing operation, those disclosed in JP 2012-128037 A and JP 2012-128189 A are known. By these techniques, the temperature rise curve of the fixing member is estimated in advance. The temperature rise curve is the curve that represents the relationship that is established between the time of the resumption of the temperature increasing operation by the fixing unit and the temperature of the fixing member at the time of the resumption when the following condition is satisfied: “by the time when the fixing unit thermally fixes a toner image, the temperature of the fixing member has reached the target value that should be maintained at that time”. By the above techniques, the temperature of the fixing member is monitored while the temperature increasing operation of the fixing unit is suspended, and the temperature increasing operation of the fixing unit is resumed at the time when the point at the coordinates representing the combination of the current time and the temperature of the fixing member is located on the temperature rise curve.
JP 2012-128189 A also discloses the following technique. By this technique, the temperature of the fixing member is first measured while the temperature increasing operation of the fixing unit is suspended, and the curve indicating a temporal variation in the temperature of the fixing member, or the temperature drop curve, is estimated from the measured value of the temperature of the fixing member. The coordinates of the intersection point between the temperature rise curve and the temperature drop curve are then predicted, and the temperature increasing operation of the fixing unit is resumed at the time indicated by the intersection point.
By these techniques, the estimation of the temperature rise curve is based on an experiment or measurement that is carried out prior to actual printing operations. Specifically, in a case where the estimated shape of the temperature rise curve is approximated by a straight line, a predicted value of the temperature rise rate that is represented by the tilt of the straight line is determined through an experiment. JP 2012-128189 A also discloses a technique by which the temperature rise rate of the fixing member is measured at the time of activation of the image forming apparatus, and the measured value is set as a predicted value.
In recent years, there is an increasing demand for power saving features in electronic products in general. Image forming apparatuses are also expected to further reduce power consumption. To satisfy the demand, a further reduction in power consumption by the fixing unit is effective as described above.
So as to further reduce the power consumption by the fixing unit while preventing printing quality degradation due to insufficient thermal fixing, the accuracy of temperature rise curve estimation should be made even higher, and the temperature increasing operation should be resumed at a more accurate time.
By the techniques disclosed in JP 2012-128037 A and JP 2012-128189 A, however, it is difficult to further increase the accuracy of the temperature rise curve estimation. This is because the experiment and the like used in predicting the temperature rise rate do not reflect the state of the fixing unit that affects the amount of heat transferred to and from the fixing member, or the internal conditions of the fixing unit.